This invention relates to video display units (VDUs) for displaying data.
A conventional VDU uses a television raster to display date on a cathode ray tube (CRT). It has been found that the video signal circuits produce electromagnetic radiation which can sometimes be picked up by an eavesdropper using a conventional broadcast TV receiver. One way of overcoming this problem is to provide electromagnetic shielding for the VDU, but this can be very expensive. Another possible solution is suggested in "Electromagnetic Radiation of Information by Video Display Units", W. vanEck et al, Securicom 85 Symposium, Cannes, 6-8 Mar. 1985. This suggests scanning the faster lines in a random order e.g. 7,25,1,199 . . . instead of the conventional 1,2,3,4 . . . . Only if the eavesdropper knows the sequence will he be able to reconstruct the data display. However, this method requires the ability to move the CRT beam between any pair of lines on the CRT screen, and to allow it to settle in the new position with an accuracy of one part in 2000, within the line flyback period, i.e. 5-12 microseconds. This cannot be achieved with conventional scanning circuits and requires very costly special circuits.
The object of the present invention is to provide a way of preventing, or at least reducing the possibility of, eavesdropping which does not suffer from these problems.